In order to better understand the origin of ageing on driven micropiles in sand, a series of field pull-out tests on small diameter driven piles were carried out in Dunkirk. The micropiles were about 51 mm in diameter, 8 mm wall thickness and 2 m embedment. The micropiles were made of different steel types (mild steel or stainless steel) with different shaft roughness. Sand characteristics, field penetrometer tests and pile tests are summarized. Tension tests were performed at different intervals of time after micropiles installation in attempt to investigate the influence of: (i) physiochemical effects, by testing both stainless and mild steel piles; (ii) the early stages of ageing. A significant increase of capacity was observed in the case of mild steel micropiles, regardless of the shaft roughness, while no ageing effects were observed on stainless steel micropiles. Some micropiles have been pre-loaded in compression at different energy levels. These results suggest the great influence of physiochemical effects (corrosion) for small diameter piles driven in sand. In the case of a prior compressive plastification, the loss of the benefit of ageing is obvious.